Monday, February 4, 2013

A new paper published in Nature Climate Change finds tree-ring reconstructions of temperature, such as Mann's infamous hockey stick, "underestimate climate fluctuations of, for example, air temperature," due to data complicated by "the climate of past years and other factors like tree age" and precipitation. "Our results point to uncertainties in the global climate system that were previously not recognized," says David Frank, co-author of the study.

Section of annual tree-rings of Scotch pine (Pinus sylvestris) from Valais (Sitzerland). The tree-rings consist of thick- and thin-walled cells and can be wide or narrow depending on the weather, species, tree age and site. (Credit: WSL)

Feb. 1, 2013 — Climate signals locked in the layers of glacial ice, preserved in the annual growth rings of trees, or fingerprinted in other so-called proxy archives such as lake sediments, speleothems, and corals allow researchers to quantify climate variation prior to instrumental measurements. An international research team has now investigated hundreds of these proxy records from across the globe and compared them with both simulations of the Earth’s climate and instrumental measurements of temperature and precipitation.

Climate extremes not always recognized in proxy archivesThe scientists learned that these proxy archives provide an incomplete record of climate variation. The annual width or density of tree-rings is not only influenced by temperature while the ring is developing, but also from the climate of the past years and other factors like tree age. This makes it difficult to extract pure temperature signals from these natural archives. Importantly, the researchers found out that proxy data underestimate climate fluctuations of, for example, air temperature over the land surface where large year-to-year variability is common. In contrast, long-term trends in precipitation tend to be exaggerated by the proxy records. These findings indicate that the proxy data often result in a “blurry picture” of climate variation. The researchers were able to conclude from their work that short-term extreme climate events, such as individual years with hot summers, are not well captured by the proxy reconstructions.Temperature trends can’t be used to understand rainfallInvestigations on the individual factors and processes fingerprinted in tree-ring, ice-core and speleothem records are needed to develop a more accurate history and understanding of the climate system. The authors explicitly warn that proxy records that predominately reflect temperature variation should not be used to make conclusions about precipitation change and vice-versa. "Our results point to uncertainties in the global climate system that were previously not recognized," says David Frank, co-author of this study. He continues: "This might be surprising because we know more about the Earth’s climate now than say 20-years ago. Part of the scientific process is to confront and uncover these unknowns while developing climate reconstructions." There is still a lot of basic research needed to reduce uncertainties about how the Earth’s climate system operated prior to the industrial era and how it may operate in the future.

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External forcing and internal dynamics result in climate system variability ranging from sub-daily weather to multi-centennial trends and beyond1, 2. State-of-the-art palaeoclimatic methods routinely use hydroclimatic proxies to reconstruct temperature (for example, refs 3, 4), possibly blurring differences in the variability continuum of temperature and precipitation before the instrumental period. Here, we assess the spectral characteristics of temperature and precipitation fluctuations in observations, model simulations and proxy records across the globe. We find that whereas an ensemble of different general circulation models represents patterns captured in instrumental measurements, such as land–ocean contrasts and enhanced low-frequency tropical variability, the tree-ring-dominated proxy collection does not. The observed dominance of inter-annual precipitation fluctuations is not reflected in the annually resolved hydroclimatic proxy records. Likewise, temperature-sensitive proxies overestimate, on average, the ratio of low- to high-frequency variability. These spectral biases in the proxy records seem to propagate into multi-proxy climate reconstructions for which we observe an overestimation of low-frequency signals. Thus, a proper representation of the high- to low-frequency spectrum in proxy records is needed to reduce uncertainties in climate reconstruction efforts.